Vehicles have been developed to perform an engine stop when idle-stop conditions are met and then to automatically restart the engine when restart conditions are met. Such idle-stop systems enable fuel savings, reduced exhaust emissions, reduced vehicle noise, and the like.
During an engine restart, a target air-to-fuel ratio profile may used to control the generated torque and improve engine startability. Various approaches may be used for air-to-fuel ratio control at the engine start. One example approach is illustrated by Kita in US 2007/0051342 A1. Therein, angular speed information from a crankshaft, during an engine run-up, is used to identify torque deviations from a desired torque profile, as caused by air-to-fuel ratio fluctuations. Fueling adjustments are then used to correct for the air-to-fuel ratio deviations.
However, the inventors herein have identified a potential issue with such an approach. Cylinder-to-cylinder air-to-fuel ratio variations during engine cranking may not be sufficiently addressed with the adjustments of Kita. Specifically, the deviations, and corresponding corrections, are learned in Kita as a function of engine speed-load conditions. However, fueling errors for a particular cylinder may be more tied to the combustion event number from the time the engine is restarted. Since the corrections learned by Kita may not be properly parsed, even when tracked on a per-cylinder basis, the fueling errors may cancel out over time. As a result, cylinder-to-cylinder air-to-fuel ratio deviations may occur during engine cranking, in particular, in vehicles configured to start and stop frequently in response to idle-stop conditions. These deviations may then cause the engine speed to flare or undershoot, leading to NVH issues during engine cranking. As such, this may degrade engine startability and reduce driver feel.
Thus in one example, some of the above issues may be at least partly addressed by a method of controlling an engine. In one embodiment, the method comprises, during an automatic engine restart from an engine stop, correlating fueling errors to engine cylinders based on a number of combustion events from a first combustion event and a cylinder identity. Herein, the fueling errors may be identified based on crankshaft speed fluctuations. In this way, cylinder-specific variations may be better learned and compensated when they are tied to the combustion firing order taking into account the first cylinder to fire during the start. For example, the method may identify the first combustion of the engine restart, before which no cylinders have combusted, and then track air-to-fuel ratio errors according to the order of combustion from that first combustion event. In this way, even when a different cylinder is the first to fire, proper compensation can be provided. Note that air-to-fuel ratio errors may be based on a variety of factors alternatively to crankshaft speed fluctuations. Further, there are various approaches to identify air-to-fuel ratio errors from crankshaft speed fluctuations, and such errors can further be based on exhaust air-to-fuel ratio information.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.